1. Field of the Invention:
The present invention relates to an apparatus and method for bending a workpiece.
2. Description of the Related Art:
A technique for bending a workpiece is known from Japanese patent publication No. 6-269861, a main part of which is shown in FIG. 13. As shown in FIG. 13, a rotary table 1 is connected to a die 2. The rotary table 1 is rotatable around the vertical axis at a central point thereof. The die 2 is used to bend a workpiece W facing a surface of the die 2. A holding member 3 comprises a first holding member 31 and a second holding member 32. The first holding member 31 is fixed on the rotary table 1 and holds one end portion of the workpiece W. The second holding member 32 holds the other end portion of the workpiece W.
The rotary table 1 is connected to a rotary driver 4. The rotary driver 4 is used to rotate the rotary table 1 and bend the workpiece W in a horizontal direction. The second holding member 32 is connected a load cell 8 which is further connected to a twisting driver 9. The twisting driver 9 is used to apply a rotational force to the second holding member 32 and twist the workpiece W around an axis thereof. The twisting driver 9 is fixed on an up-down table 52 of an up-down driver 5. The up-down driver 5 is used to move the up-down table 52 in a vertical direction and bend the workpiece in the vertical direction. The up-down driver 5 is connected to a slide table 62 of a forward and backward driver 6. The forward and backward driver 6 is used to move the slide table 62 in the forward and backward direction and stretch the workpiece W by applying a stretching force thereto.
The die 2 has a side wall surface and an upper wall surface. The side wall surface and the upper wall surface are respectively formed to a desired arc.
The rotary driver 4, the up-down driver 5, the forward and backward driver 6, and the twisting driver 9 are electrically connected to and controlled by an NC controller 7 having a memory. Information detected by the load cell 8 is transmitted to a calculating device 82 and a result of the calculation based on the information of the load cell 8 is transmitted to the NC controller 7.
When an operation is started, each driver 4, 5, 6 is driven respectively according to a command signal outputted by the NC controller 7. The rotary table 1 is rotated by a driving force from the rotary driver 4 and the workpiece W is bent along the side wall surface of the die 2. The second holding member 32 is moved upward by a driving force from the up-down driver 5 and the workpiece W is bent along the upper wall surface of the die 2. Further, the second holding member 32 is rotated to twist the workpiece W by applying the driving force from the twisting driver.
In case of bending a stiff workpiece, it is necessary to bend the workpiece while applying a tension force to the workpiece to prevent the workpiece from being broken. For that reason, the second holding member 32 stretches the workpiece by applying a stretching force via the forward and backward driver 6.
Since each driver 4, 5, 6, 9 is operated as described above, the workpiece W is bent to form a desired shape.
According to this method of bending the workpiece, when the second holding member 32 is moved in the vertical direction (Y-axis direction shown in FIG. 13), the portion of the workpiece W held by the second holding member 32 is inclined relative to the horizontal direction (at X-axis direction shown in FIG. 14), which is the direction the workpiece is stretched by the forward and backward driver 6 in the vertical plane. Further, when the rotary table 1 is rotated, the portion of the workpiece W held by the second holding member 32 is inclined relative to the horizontal direction in the horizontal plane. Therefore, the direction of the tension force acting on the workpiece is different from the direction of the stretching force applied to the workpiece W by the forward and backward driver. Since the direction of the tension force is different from the direction of the stretching force, not all of the stretching force is transmitted to the workpiece as a tension force. Further, an inclination angle made by the direction of the tension force and the direction of the stretching force is variable in the bending process. Since the amount of spring back after bending is related to the tension force, an error in the dimension of the workpiece generated by the spring back after bending become larger.